The present invention generally pertains to detection of the presence of an identification signal within a composite signal, and is particularly directed to avoiding falsely recognizing as identification signals, other sharp transitions in the composite signal.
The present invention is useful for detecting sidelobe identification signals in a system in which sidelobe signals are modulated by an identification signal so that the sidelobe signals can be discriminated from the main beam signals by detection of the identification signal. A prior art system utilizing a sidelobe identification signal for sidelobe descrimination is described in the U.S. Pat. No. 4,266,226 to McNaul et al.
In one prior art system, the presence of an identification signal in a sampled composite signal is detected by delaying the composite signal by one-half the frequency of the identification signal and comparing the delayed composite signal with the undelayed composite signal. The composite signal is delayed in this prior art system by a sample-and-hold circuit. This prior art system uses a simple window detector to detect the presence of the identification signal by determining whether a change in voltage has occurred. However, this prior art system is sensitive to all sharp signal transitions that occur within the half-cycle period of the identification signal, and thereby falsely recognizes as identification signals other sharp transitions in the signal being sampled. As a result, a sharp transition is a main beam signal could be falsely recognized as a sidelobe identification signal.